The present invention relates to an ND (neutral density) filter including a plastic sheet substrate and a deposited film on the substrate, and also relates to an aperture device including the ND filter.
An aperture device used in a lens optical system such as a still camera and video camera includes an ND filter formed in a sheet to prevent hunting of an aperture blade and low resolution by diffraction at a small diaphragm opening. Lately, a resolution of a camera has been increased and a size of a camera has been decreased. In such cases, when an ND filter having a uniform light transmittance is used, it may be difficult to effectively prevent the diffraction due to a steep change in a light intensity when the filter enters the aperture opening. Accordingly, it has been proposed to provide an ND filter with various light transmittances changed stepwise.
An ND filter is generally disposed to be movable in a lens optical system. Therefore, it is necessary to use a thin and light material for the ND filter. The ND filter is generally formed in a plastic sheet, so that it is easy to form in a desired shape corresponding to an aperture opening. Such ND filters include a filter in which a plastic material containing an organic dye or pigment is formed in a sheet to provide a desired optical characteristic, and a filter in which a film is deposited on a surface of a transparent plastic material to provide a desired optical characteristic.
In particular, the filter having the deposited film exhibits a high anti-reflection effect at a surface of the filter through a combination of deposited layers. Thus, the filter is effective in reducing a ghost in the lens optical system. Further, it is possible to provide a filter having areas with various light transmittances changed stepwise, so that the film can reduce transmittance without a steep change in the transmittance, thereby preventing the diffraction more effectively.
As a conventional filter, Japanese Patent Publication (KOKAI) No. 10-133253 has disclosed a ND filter in which a deposited film is formed on a plastic sheet. The ND filter uses polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) as the plastic sheet, and titanium oxide and aluminum oxide are deposited on the plastic sheet as the deposited films.
In the conventional ND filter, the plastic sheet is formed of PET and PEN having a relatively large thermal shrinkage factor (thermal shrinkage factor is equal to 0.5%/150° C.·30 min). Therefore, when the sheet is exposed to a high-temperature atmosphere for a long period of time in a deposition step, a surface of the sheet tends to have wrinkles due to thermal shrinkage. In a severe case, the deposited film may be peeled off from the plastic sheet due to a difference in thermal shrinkage factors between the deposited film and the plastic sheet.
It is known that a plastic sheet is susceptible to heat. Accordingly, a temperature of a vacuum deposition chamber is maintained as low as possible. However, there is a case in which a plastic sheet is heated up to about 120° C. due to heat when an electron gun heats and melts a deposited material or when the plastic sheet is heated to securely adhere the deposit film to obtain a desired light transmittance. In production of the ND filter with various light transmittances changed stepwise, it is necessary to repeat the deposition step several times, thereby exposing the plastic sheet in a high-temperature environment for a longer period of time.
In the conventional ND filter described above, the deposited film is formed of titanium oxide as a layer for absorbing light. When the titanium oxide is exposed to the atmosphere from the high temperature environment in the deposit chamber, there is a risk in which the titanium oxide is oxidized and the light transmittance thereof relative to a certain wavelength changes. It is necessary for the ND filter to possess a uniform transmittance over the visible light region. Therefore, the oxidation must be strictly controlled. In production of the ND filter with the light transmittance changed stepwise, it is necessary to take the filter out from the chamber to change a mask, thereby increasing a possibility of the oxidation and making it difficult to strictly control the oxidation.
In view of the problems described above, an object of the present invention is to provide an ND filter formed of a plastic sheet with a low thermal shrinkage factor as a substrate to prevent wrinkle even after long exposure to a high temperature environment. The filter has a deposited film with good oxidation resistance to obtain a uniform transmittance over the visible light region. It is also easy to control the oxidation in the deposition step.
Further objects and advantages of the invention will be apparent from the following description of the invention.